Coal combustion fly ash is a useful additive in concrete due to its pozzolanic property—i.e., its ability to react with calcium in concrete mixes and thus contribute to the formation of the cementitious matrix. Through this mechanism, fly ash serves as a partial replacement for Portland cement, yielding cost savings as well as a variety of concrete property enhancements, which may include reduced permeability, improved workability, increased long-term strength, and reduced threat of long-term failure due to alkali-silica reaction.
Carbon in fly ash is an undesirable contaminant in cases where fly ash is used as a pozzolanic admixture in concrete. Carbon adsorbs the surfactants or “air entraining admixtures” used in concrete making them unavailable for their primary function. U.S. Pat. No. 6,136,089 and publication (Gao et al., 2001) teach the use of ozone to chemically passivate the surfaces of carbon in ash in order to improve air entrainment behavior. This process leaves the carbon in place, but adds surface oxygen complexes whose presence inhibits adsorption of the surfactant.
A practical problem with this recycling technology is the tendency of residual carbon in ash to interfere with the air entrainment process in concrete. Porous carbon adsorbs the chemical surfactants (air entraining admixtures, or AEAs) used to generate and stabilize a micro-void system in concrete pastes. Without a sufficient network of sub-millimeter air bubbles, concrete fails under internal pressure generated by the freezing and expansion of trapped residual water. About two-thirds of the concrete in North America is air entrained, and this surfactant adsorption phenomenon is the primary driving force for national and regional regulations limiting the carbon content of ash used in concrete.
Ash samples from the field show great variability in the extent to which they adsorb AEAs. Recent work has identified the following four primary factors governing ash adsorptivity: (1) the mass fraction carbon, (2) the total surface area of the carbon, (3) the accessibility of that surface, as governed by particle size and pore size distribution, and (4) the carbon surface chemistry. The inorganic fraction of ash is found to play a very minor role in AEA adsorption.
The role of carbon surface chemistry is particularly apparent from the behavior of ash during thermal oxidation in air. Introduction of surface oxides by exposure to air at 350-450° C. has been observed to significantly reduce subsequent AEA adsorption without consuming a measurable amount of carbon. In contrast, treatment in inert gas at temperatures sufficient to drive-off many pre-existing surface oxides (900° C.) has been observed in increase adsorption. Commercial carbon blacks subjected to surface oxidation processes have also been observed to be less adsorptive than non-treated varieties. Both of these observations suggest that oxide-free carbon surfaces are the most active for adsorption of surfactants. The important role of a non-polar (oxide-free) surface is not surprising, as polar functionalities are already abundant in concrete pastes (on inorganic fly ash particles, cement particles, aggregate particles, and in the aqueous solution), whereas the only non-polar components are air bubbles and a portion of the carbon surface. It is likely that the non-polar portions of the carbon surfaces compete directly with the air bubbles for the non-polar portions of the surfactant molecules. This insight suggests that the deleterious effect of carbon could be suppressed by intentional oxidation of the largely non-polar carbon surfaces.
Possibilities for intentional surface oxidation include dry and wet chemical methods. Many wet oxidation agents have been used to surface treat other carbon materials, including HNO3, H2O2 CH3COOH, and (NH3)2S2O8, but for the treatment of ash these wet processes would have practical disadvantages, including high drying costs, and potential problems with self-cementation or loss of pozzolanic activity. Dry oxidation in air requires temperatures above about 300° C., and is not likely to offer advantages over commercial combustion-based processes, which remove the carbon altogether while operating at only modestly higher temperatures. For these reasons, this patent focuses on ozone, O3, as an oxidant capable of attacking carbon surfaces in ash in the dry state and at ambient temperature.
Attention is brought to the fact that Gao et al in Fuel, Vol. 80 (2001), pages 765 to 768 have published an article related to the ozonation of fly ash.
Prior art U.S. Pat. No. 6,136,089 to Hurt et al teaches a method for ozonating fly ash in order to deactivate carbon in fly ash. The method of the patent uses 500 ppm ozone in air at 0.9 lit/min. in a packed or fixed bed treatment. Treatment of fly ash in a pneumatic conveyor tube is taught. The patent also teaches the use of a fluidized bed to ozonate fly ash. Ozonation of fly ash while the fly ash is stored in a silo is also taught. However, what is not taught by the patent, are unobvious, favorable processing conditions for the economic ozonation of fly ash. The favorable processing conditions are attained by making certain that there are favorable contact conditions between the ozone and the fly ash to be treated. The herein disclosed invention is designed to produce a process wherein the fly ash is optimally treated with ozone. The process assures that over saturation does not occur with its attendant waste of ozone. For example, data presented in the patent clearly shows that sustained upflow through a stationary bed leads to over saturation of the bottom portion of the bed with the attendant excessive use of ozone and the unnecessary gasification of carbon. Large-scale utility application of the process of U.S. Pat. No. 6,136,089 involving deep beds, and fixed-bed contacting schemes will be uneconomic. For this reason, the ash bed must be continuously mixed or transported by mechanical or aerodynamic means to prevent prolonged contact of any part of the bed with fresh ozone. Further, the amount of ozone relative to the fly ash in the herein disclosed invention is controlled; e.g., gm-ozone/kg-carbon. This is a basis for the herein disclosed invention. Embodiments of the invention are presented below.